1. Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof. More specifically, the present invention relates to a semiconductor device and manufacturing method thereof in which a nitrided oxide film is used as a gate insulating film.
2. Description of the Background Art
Demand of semiconductor devices has been rapidly increasing along with remarkable spread of information equipment such as computers. As for the function, devices having large storage capacity and capable of high speed operation are in demand. Accordingly, technical efforts have been made to realize higher degree of integration, higher speed of response and higher reliability of the semiconductor devices.
A DRAM (Dynamic Random Access memory) has been generally known as one of the semiconductor devices that allows random input/output of memory information. The DRAM includes a memory cell array as a storage region storing a number of pieces of memory information, and peripheral circuitry necessary for external input/output.
The memory cell array is formed by a plurality of memory cells each storing a unit memory information arranged in a matrix of rows and columns. The memory cell generally includes one MOS transistor.
FIG. 45 is a cross section of an MOS transistor used in a conventional DRAM. Referring to FIG. 45, n type source.multidot.drain regions 202a and 202b are formed in a p type silicon substrate 201. A region between the source.multidot.drain regions 202a and 202b is a p type channel region 201a. A gate electrode 204 is formed on channel region 201a with a gate insulating film 203 formed of a silicon oxide interposed.
When the MOS transistor structured in this manner is operated, a voltage is applied to gate electrode 204. Then, channel region 201a is inverted to n type, and therefore current flows through the source.multidot.drain regions 202a and 202b.
When the size of the MOS transistor is reduced, thickness of gate insulating film 203 is reduced, and the distance between the source.multidot.drain regions 202a and 202b is also reduced. Even in that case, the voltage applied to gate electrode 204 and the voltage applied to source.multidot.drain regions 202a and 202b are not much different from those used conventionally. Therefore, when miniaturized, electric field in the longitudinal direction of channel 201a (longitudinal direction of FIG. 45) becomes higher. This lowers effective mobility .mu..sub.eff of electrons in channel region 201a, degrading drivability of the transistor.
Further, since electric field in the lateral direction of the channel region 201a (direction from source.multidot.drain region 202a to source.multidot.drain region 202b) becomes higher, and carriers (electrons) are accelerated, promoting entrance of carriers into gate insulating film 203. This undesirably makes shorter the hot carrier life of the transistor.
Further, since gate insulating film 203 is made thinner, electric field attains higher also in gate insulating film 203. Accordingly, insulation of gate insulating film is degraded, resulting in sharp lowering of dielectric breakdown life T.sub.bd against TDDB (Time Dependent Dielectric Breakdown) of the transistor.
In order to solve these problems, a transistor using a silicon nitrided oxide as the gate insulating film has been proposed in Japanese Patent Publication No. 7-28041. FIG. 46 is a cross section of the transistor using a silicon nitrided oxide as the gate insulating film. In the transistor shown in FIG. 45, silicon oxide was used for the gate insulating film, while in the transistor shown in FIG. 46, silicon nitrided oxide 103 is used as the gate insulating film.
Other than this point, an element isolating insulating film 104 is formed on a silicon substrate 101, and an interlayer insulating film 107 is formed on element isolating insulating film 104. Source.multidot.drain regions 106 are formed at the surface of silicon substrate, and a gate electrode 105 is formed with silicon nitrided oxide 103 interposed, on the surface of silicon substrate 101. An interlayer insulating film 107 is formed to cover gate electrode 105, and an aluminum electrode 108 reaching source.multidot.drain regions 106 is formed on interlayer insulating film 107.
In the semiconductor device structured as described above, effective mobility .mu..sub.eff of electrons is improved. However, the problems of degraded hot carrier life and dielectric breakdown life are not solved.
U.S. Pat. No. 5,237,188 also discloses a transistor utilizing a nitrided oxide as the gate insulating film. However, this transistor does not solve the problems of degraded hot carrier life and dielectric breakdown life, either.